Different Groups Of Microorganisms


  • Generally microbes can be divided into two categories: the cellular microbes (or organisms) and the acellular microbes (or agents).
  • In the cellular camp we have the bacteria, the archaea, the fungi, and the protists (a bit of a grab bag comp protozoa, slime molds, and water molds).
  • Cellular microbes can be eitherwhere one cell is the entire organism, or multicellular, where hundreds, the even billions of cells can make up the entire organism.
  • In the acellular camp we have the viruses and other infectious agents, such as prions and viroids.
  • the traditional definition is modified to describe microbes as fairly simple agents.
  • organisms that are not highly differentiated, meaning even the multicellular microbes are composed of cells that can act independently, there is no set division of labor.
  • If you take a giant fungus and chop half the cells off, the remaining cells will continue to function unimpeded.
  • Versus if you chopped half my cells off, well, that would be aproblem.
  • Multicellular microbes, even if composed of billions of cells, are relatively simple in design, usually composed of branching filaments.
  • It is also acknowledged that research in the field of microbiology will require certain common techniques, largely related to the size of the quarry.
  • Its Because microbes are so small and there are so many around, it is important to be able to isolate the one type that you are interested in.
  • This involves methods of sterilization, to prevent unwanted contamination, and observation, to confirm that you have fully isolated the microbe.
  • Since size is a bit of theme in microbiology, let us talk about actual measurements. How small is small? The cellular microbes are typically measured in micrometers (um).
  • A typical bacterial cell (let us say E. coli) is about 1 um wide by 4 um long.
  • A typical protozoal cell (let us say Paramecium) is about 25 um wide by 100 um long.
  • There are 1000 um in every millimeter, so that shows why it is difficult to see most microbes without assistance.
  • (An exception would be a multicellular microbe, such as a fungus. If you get enough cells together in one place, you can definitely see them without a microscope!)


  • The small size of microbes definitely hindered their discovery. It is hard to get people to believe that their skin is covered with billions of small creatures, if you cannot show it to them. “Seeing is believing, that is what I always say.
  • In microbiology, there are two people that are given the credit for the discovery of microbes. Or at least providing the proof of their discovery, both around the same time period:

ROBERT HOOKE (1635-1703)

  • Robert Hooke was a scientist who used a compound microscope, or microscope with two lenses in tandem, to observe many different objects.
  • He made detailed drawings of his observations, publishing them in the scientific literature of the day, and is credited with publishing the first drawings of microorganisms.
  • In 1665 he published a book by the name of Micrographia, with drawing of microbes such as fungi, as well as other organisms and cell structures.
  • His microscopes were restricted in their resolution, or clarity, which appeared to limit what microbes he was able to observe.



  • Antony van Leeuwenhoek was a Dutch cloth merchant, who also happened to dabble in microscopes. He constructed a simple microscope (which has a single lens), where the lens was held between two silver plates. Apparently he relished viewing microbes from many different sample types – pond water, fecal material, teeth scrapings, etc.
  • He made detailed drawings and notes about his observations and discoveries, sending them off to the Royal Society of London, the scientific organization of that time.
  • This invaluable record clearly indicates that he saw both bacteria and a wide variety of protists.
  • Some microbiologists refer to van Leeuwenhoek as the “Father of Microbiology” because of his contributions to the field.


  • Classification of organisms, or the determination of how to group them, continually changes as we acquire new information and new tools of assessing the characteristics of an organism.
  • Currently all organisms are grouped into one of three categories or domains:
  • Bacteria, Archaea, and Eukarya. The Three Domain Classification, first proposed by Carl Woese in the 1970s, is based on ribosomal RNA (rRNA) sequences and widely accepted by
  • scientists today as the most accurate current portrayal of organism relatedness.


  • The Bacteria domain contains some of the best known microbial example Most of the members are unicellular, cells lack a nucleus or any other organelle.
  • Most members have a cell wall with a particular substance known as peptidoglycan (not found anywhere else but in bacteria.
  • The humans are intimately familiar with manymembers, since they are common in soil, water, our foods, and our own bodies.


  • Archaea is a relatively new domain, since these organisms used to be grouped with the bacteria.
  • There are some obvious similarities, since they are mostly unicellular and cells lack a nucleus or any other organelle.
  • They have completely different cell walls that vary markedly in composition (but notably lack peptidoglycan) and their rRNA sequences have shown that they are not closely related to the Bacteria at all.
  • In fact, they appear to be more closely related to the eukaryotes! These organisms are found in soil, water, even sometimes in the human body.
  • They are also found in some very extreme environments on Earth – very cold, very hot, very salty, very pressurized, very acidic, earning them the commonly used name “the extremophiles,” or extreme-loving organisms


  • The Eukarya Domain includes many non-microbes, such as animals and plants, but thereare numerous microbial examples as well, such as fungi, protists, slime molds, and water molds.
  • The eukaryotic cell type has a nucleus, as well as many organelles, such as mitochondria or endoplasmic reticulum.


  • Viruses are not part of the Three Domain Classification, since they lack ribosomes and therefore lack rRNA sequences for comparison.
  • They are classified separately, using characteristics specific to viruses. Viruses are typically described as “obligate
  • intracellular parasites,” a reference to their strict requirement for a host cell in order to replicate or increase in number.
  • These acellular entities are often agents of disease, as a result of their cell invasion.